1. Field of the Invention
The invention relates generally to voice communication and tone control systems, and more specifically relates to such systems for intra-or interplant use that operate between hazardous and safe areas. The present invention further relates to control and annunciator systems designed to operate at power levels low enough to render the system intrinsically safe and reliable enough to maintain this intrinisically safe operation even if the system malufunctions.
2. Description of the Prior Art
Communication and control systems utilizing electrical energy can cause severe explosions or fire in a hazardous atmosphere. To prevent this the standard practice is to house all active components of communciation and control systems and THE SYSTEMS interconnecting wiring in heavy, hermetically sealed housing and conduits. Any local explosion caused by the electrical equipment would then be confined within the explosion proof housing and could not cause a general explosion.
It is also possible to prevent explosions by purging the hazardous area with an inert gas, ventilation, or burning a continuous pilot. All of these systems assure safe operation by preventing the uncontrolled accumulation of a large volume of hazardous gas, thus preventing an explosion that could damage life or property.
Another method of ensuring safe operation is to limit the amount of power in the hazardous area. Signal, instrument control and annuciator systems have been designed to operate at such low power levels that electrical equipment in the hazardous area becomes intrinsically safe. By definition, an intrinsically safe system cannot release enough energy under either normal or failure conditions to cause ignition of the hazardous atmospheric mixture in its most easily ignitable concentration. The English, who pioneered in the field of energy limited circuits, report that intrinsically safe systems can be constructed either by providing energy limiting circuits for each instrument having connections to the hazardous area or by adding energy limiting devices, called barriers, in each signal or power line going to or from a hazardous area. The history of such intrinsically safe equipment is detailed in a publication entitled "The Barrier Approach to Intrinsic Safety" available as Taylor Technical Data Sheet No. TDS-20A100 from the Taylor Instrument Company, Rochester, New York.
All microphones used in telephonic communication systems operate by either generating or modulating an electrical signal. The accustical energy in the human voice must be capable of moving a mechanical part, usually a diaphragm. The moving diaphragm then controls the flow or production of electrical energy. Diaphragms flexible enough to be moved by the human voice must be thin. A thin diaphragm cannot meet the safety standards required for safe operation in hazardous areas. Thus the power levels present in the microphones of a telephone system operating in a hazardous area must be low enough to be intrinsically safe.
The carbon microphones used in ordinary telephones operate by imposing a relatively high (unsafe) voltage across a pad of carbon that is compressed and released by the movement of a thin diaphragm. As the carbon is compressed it presents a varying resistance to the flow of current, thus modulating it. If the diaphragm is punctured or the connecting cable cut, an unsafe level of electrical power will be exposed to the hazardous area's atmosphere. Because of this, carbon microphones cannot be used in hazardous areas without explosion proof protections.
Ddynamic microphones generate a very low level of electrical energy by moving a coil of wire attached to the flexible diaphragm near a magnet. This voltage output is low enough to be intrinsically safe and dynamic microphones may safely be used in hazardous areas. Unfortunately, the low voltage produced by a dynamic microphone must be amplified before it can be used in a telephone system. Because of line losses and noise, the amplifier must be located as near as possible to the microphone. Thus all telephone systems operating in hazardous areas consist of a dynamic microphone in a handset and a field amplifier mounted near the handset (for example, the system taught by U.S. Pat. No. 3,080,454). This amplifier requires a high power input (in the case of the cited reference, a 110 volt AC supply) to produce an amplified signal output strong enough to be transmitted to and through the rest of the system. Because of its high (unsafe) power requirements, the field amplifier must be housed in an explosion proof housing having covers affixed by multiple bolts, and the power and signal wiring must be enclosed in sealed conduits. These heavy and expensive provisions designed to insure explosion proof operation could still fail if a cover bolt was improperly tightened or a scratch made in the ground (i.e. fitted) surface of a cover plate during maintenance. Additionally, the connection between the intrinsically safe handset and the amplifier in an explosion proof housing could only be made through a special sealing connector.
These complex, heavy, expensive and failure prone "explosion proof" housings and cabinets were necessary because the field amplifier required high power levels to amplify weak signals from the dynamic microphone and to drive these amplified signals back to a central station or around a series circuit party line.
The prior art, in addition to disclosing high power levels in the field, usually taught operation of the individual field handsets in a series circuit or "party line." Party line operation lessened the expense and possibility of failure associated with running a plurality of parallel circuits through explosion proof conduits. Failure of any part of a party line circuit (i.e. via explosion) puts the entire communications system or some substantial part of it out of operation. In an emergency, power could not be maintained to any part of a series circuit system because this would create a possibility of causing further explosions. Examples of the present state of the art, as described above, together with types of explosion proof housings currently required for safe operation may be found in the 1974 Browne-Davies Electronics Corporation Catalog available from the company at 10600 Fallstone Rd., Houston, Tx.